Research into measles cases in Victoria has shown a number of people who were hospitalised with the disease had been vaccinated at least once in the past, but their immunity had waned over time.
Published in the Clinical Infectious Diseases journal, the research found that between 2014 and 2017, a small number of measles cases (13) had what is called secondary vaccine failure, meaning they had at least one dose of the vaccine and showed antibodies in their blood, but that protection waned and they contracted measles.
Measles is a highly infectious disease, with an estimated 110,000 deaths worldwide in 2017, mostly in children under five. It is so contagious that 90 per cent of people not immune that come into contact with a case will become infected. There is a nationally funded, safe and effective vaccine available.
The Royal Melbourne Hospital’s Dr Katherine Gibney, an epidemiologist at the Doherty Institute and the Victorian Department of Health and Human Services, said because Australia has done such a good job at eliminating measles, people who have had only one vaccine aren’t getting a natural immunity boost as the disease isn’t circulating in the community.
“If you had been vaccinated and came into contact with someone with measles you might get a little natural boost in your antibody levels,” she explained.
“Overall, in countries that have eliminated measles transmission, this is likely to emerge as a problem. There isn’t going to be an enormous number of cases, but it will be important in terms of recognising measles, because the cases are a bit different to those who aren’t immune.”
While these cases were hospitalised, the symptoms weren’t considered ‘classic measles’ – patients weren’t reporting fever, cough and runny nose, but they did have a rash.
“Normally, if people have documented receiving two doses of measles vaccine we would be confident they won’t contract measles, but that’s getting greyer – this research has demonstrated some vaccinated people are getting measles,” Dr Gibney said.
“We do have a definitive test for measles. Our message to doctors is that if you suspect measles, don’t just rely on the serology, which detects antibodies to measles, but also perform a PCR test, which detects the actual virus.”
These results also present a public health problem with transmission; researchers documented the transmission from one waning immunity case to two infant household contacts, too young for vaccination.
“In most cases, people are unaware they need the second vaccine, or they simply don’t remember if they have had one or two,” Dr Gibney said.
“Anyone who is unsure if they have had two doses of measles vaccine should see their doctor about getting an additional dose. In particular, adults born after 1965 might not have received two doses of measles vaccine during their routine childhood immunisation. The Victorian Government provides the measles vaccine free of charge for these people.
“More work needs to be done in the area of a third vaccine before we can routinely recommend this – we need to know definitively if a third booster shot will extend the immunity to measles for a lifetime.”